We present results obtained via an innovative spaceborne SAR interferometry algorithm showing that the Somma-Vesuvius volcanic complex, despite of its quiescent stage, is subject to a particular deformation process. This is characterized by a rather continuous subsidence, revealed by ERS satellite data and levelling surveys, between 1992 and 2000. These deformations are mainly localized in two zones involving the Vesuvius cone and a narrow annular area that, although not fully continuously, extends around the base of the Somma edifice. We propose an interpretation of subsidence at both sites involving joint effects of gravitational sliding and extensional tectonic stress occurring at the contact between different lithological units. Some simple elastic models show how such localized subsidence can be generated. These results shed new light on the Vesuvius dynamics and, more generally, on the link between gravitational effects of volcano loading and seismic-deformative processes, which is a subject of intense scientific debate.